Elena Saejima, a schoolgirl sister, has been mysteriously absent from school and shut herself in her home for nearly half a year. One day, her older brother discovers her masturbation habit and gives her a new toy as a gift. Watching her use it ignites his desire, and he barges into her room, forcefully initiating sex. Initially confused and resistant, Elena gradually becomes obedient to her brother's advances, eventually submitting completely. Thus begins her training as a sex slave, fulfilling her brother's every desire.